Pressure gauges are conventionally used for measuring the pressure of a gas or a liquid, such as an air pressure. Tire pressure gauges, as an example, measure the inflation pressures of vehicle tires, such information being useful for maintaining optimal tire performance and avoiding unnecessary wear. Conventional tire pressure gauges are held in the hand, and require the user to locate a tire valve, unscrew a cap from the valve, and engage the tire pressure gauge with the valve. At night, it is difficult to locate the valve. Tire valve caps are usually covered in a film of dirt, which comes off on the user's fingers when removing and replacing the cap. While some vehicles have systems for monitoring of tire pressure using tire pressure gauges with radiofrequency transmitters installed in the tires, and systems for interrogating the gauges and receiving readings installed in the vehicle, such systems are not practical to install on existing tires and vehicles.
Mechanical devices have been developed which connect to a tire valve stem, have a spring which is urged dependent on pressure in the tire, and a mechanical device, such as a ball, which is moved by the spring. Thus, tire pressure is displayed. Such devices are disclosed in, for example, U.S. Pat. No. 5,929,330 (Ford), U.S. Pat. No. 6,911,903 (Gladstone, et al.), U.S. Pat. No. 6,374,666 (Lemberger, et al.), U.S. Pat. No. 6,531,960 (Gladstone, et al.), and U.S. Pat. No. 6,588,264 (Johnson). These devices suffer from disadvantages, such as the difficulty of seeing them at night, and, when compared with modern tire pressure gauges, the relative lack of accuracy compared to pressure sensor dies.
It would be desirable for a tire pressure gauge to be easy to install on a vehicle, possible to see at night or other dark conditions, and not require physically engaging a device with a tire valve stem each time the pressure is to be read.